Thermoplastic Forming and Related Studies of the Supercooled Liquid Region of Metallic Glasses

The thermoplastic formability (TPF) of metallic glasses was found to be related to the calorimetrically measured crystallization temperature minus the glass transition temperature, Tg Tx = ΔT. Alloy development in the ZrTiBe system identified a composition with ΔT = 120 °C. Many alloys with ΔT > 150 °C and one alloy, Zr35Ti30Be27.5Cu7.5, with ΔT = 165 °C were discovered by substituting Be with small amounts of fourth alloying elements. The viscosity as a function of temperature, η(T), and time temperature transformation (TTT) measurements for the new alloy are presented and combined to create ηTT plots (viscosity time transformation) that are useful in determining what viscosities are available for a required processing time. ηTT plots are created for many alloys used in TPF in the literature and it is found that for processes requiring 60 300 s, Zr35Ti30Be27.5Cu7.5 provides an order of magnitude lower viscosity for processing than the other metallic glasses. Injection molding is demonstrated with Zr35Ti30Be27.5Cu7.5 and the part shows improved mechanical properties over die cast specimens of the same geometry. Changes of slope in η(T) measurements were observed and investigated in some quaternary compositions and found to be present in ternary compositions as well. Traditionally metallic glasses show a single discontinuity in heat capacity at the glass transition temperature. Alloys with the changes in slope of η(T) were found to show two discontinuities in heat capacity with the changes in slope of η(T) roughly correlating with the observed Tg values. These two Tg values were assumed to arise from two glassy phases present in the alloy. Further heat capacity analysis found systematic trends in the magnitude of the heat capacity discontinuities with composition and the single phase compositions of a metastable miscibility gap were discovered.